A charging device for charging secondary batteries which are connected to one another in series in a multistage style is known.
A charging device having an overcharge protecting circuit for preventing overcharge to secondary batteries is known as the above type charging device. According to the overcharge protecting circuit, discharge route circuits each of which contains a discharge resistor are provided to the respective secondary batteries in parallel, and when overcharge of any secondary battery is detected, the secondary battery whose overcharge is detected is connected to the corresponding discharge route circuit and made to forcedly discharge, and charge current to flow into the secondary battery concerned is made to bypass the secondary battery concerned and flow into the discharge route circuit (for example, see Patent Document 1).
By providing the overcharge protecting circuit to the charging device, the following advantage is achieved in addition to the protection from the overcharge. That is, when secondary batteries which are difficult to be manufactured so that characteristics thereof such as internal resistance values under charge or the like are coincident with one another like lithium polymer batteries or the like are connected in series in a multi-stage style to charge the secondary batteries, the overcharge protecting circuit functions as a balance circuit for balancing the battery voltages of the respective secondary batteries so that the battery voltages are fitted to a fixed upper limit value. Therefore, it is possible to charge the secondary batteries with compensating dispersion in characteristic among the second batteries. Furthermore, when a secondary battery in which the battery voltage (internal resistance) under charging greatly varies due to aging degradation is charged, the upper limit value of the battery voltage is kept to a fixed value irrespective of the internal resistance of the secondary battery between the charging when use of the secondary battery is started and the charging between when aging degradation progresses to some extent, and thus the secondary battery can be charged with compensating the aging degradation.
Patent Document 1: JP-A-10-50352